Prior-art communication systems operating according to the IEEE 802.11 specification handle broadcast (BC) and multicast (MC) messages differently from unicast messages. For a unicast message the Association Identifier (AID) for the Communication Unit (CU) is mapped to its unicast Medium Access Control (MAC) address and used in the Traffic Information (TIM) frame to uniquely identify a buffered unicast message. An AID of 0 is used for BC as well as MC frames as these are addressed to a multicast group. This particular way to deliver the MC/BC messages to the CU reduces the battery life of the CU. Typical examples of multicast groups are groups of devices that receive a common video stream, a group of devices that is in a phone conference, or a group of devices that is in a multicast session such as a dispatch group call. If one uses multicast for the CUs in one of such multicast groups using the current defined MC/BC delivery mechanism in 802.11, the battery life of the other CUs in other multicast groups can be impacted considerably, even if those other groups are not receiving any multicast traffic.
The primary reason for the loss of battery life is that when an access point delivers a MC message with an AID of 0, all sleeping CUs (i.e., CUs operating in a battery saving mode) are awakened until they can determine from the message content whether they belong to the particular multicast group for which there is a MC message. Those CUs that do not belong to the particular multicast group are being awakened for no good reason and will have wasted power unnecessarily.
Thus, what is needed is a method and apparatus for sending a multicast message. The method and apparatus will preferably solve the power-wasting problem just described.